Just fitted Milltek Sport mufflers to my 996.1. Fairly muted on idle & moderate rpm, but barks on rev/overrun. Will be more aggressive with 100/200 cell cats. Quality exhaust and made in the UK. Well known euro parts supplier has 10% discount (not sure on 997 kit).
I have a C2S and went for the gundo (I think it's called hack) at £200 I thought it was well worth trying before I splashed out and it sounds great, no motorway boom and very nice under acceleration.